Power switches in traditional pulsewidth modulated (PWM) converters such as switched-mode power supplies are operated in hard switching conditions. Switched-mode power supplies are often referred to as converters, power converters, switched converters, switched power converters, and switch-mode power converters and, as such, any reference to any one of them in the following shall be taken to be a reference to all of them. During the turn-on and turn-off switching processes, the devices have to withstand high voltage and current simultaneously, resulting in high switching losses and stresses. The classical method of reducing switching losses, dv/dt, di/dt, and stresses is to use dissipative snubbers. However, dissipative snubbers produce undesirable power losses, thus limiting their application to low power or low frequency converters.
In order to overcome switching loss and enable high-frequency operation, several active snubbers that utilize soft-switching techniques have been proposed. These are operated during the short switching time to perform zero-voltage-switching (ZVS) or zero-current-switching (ZCS). The main goal of the active snubbers is to maintain advantages provided by PWM and resonant converters. The former ones have fixed-frequency operation with square current and voltage while the latter ones have low switching losses. However, such merits are often offset by requiring additional switch and control circuitry, limited operating range, and high voltage/current stresses on the switches. Due to the presence of an additional switch, the switching losses will also be increased.
Passive snubbers remain attractive alternatives as they are generally easy to design and require fewer components. A typical passive snubber consists of two parts: a turn-on snubber and a turn-off snubber. The turn-on snubber limits the rate of rise of the current through the switch and allows the voltage across it to drop before its current starts increasing. The turn-off snubber limits the rate of rise of the voltage across the switch after it is turned off. The switch is made to turn on with near ZCS and turn off with near ZVS, resulting in reduced switching losses.
A snubber has to perform two processes, namely energy absorbing and energy resetting/recirculating. The total durations taken for these two processes determine the minimum and maximum duty time of the switches. As shown in FIG. 1, the simplest form of the energy absorbing circuit for the turn-on snubber is an inductor (Ls) in series with the switching device (S) while the one for the turn-off snubber is a capacitor (Cs) in parallel with the switching device. The diode D1 provides polarized charging of Cs when the switching device is turned off, and avoids direct discharging of Cs when the switching device is turned on. Most snubber structures therefore distinguish themselves from others by the difference of their energy resetting circuits.
The simplest form of the energy resetting circuit is based on using a resistor as a dissipative energy resetting circuit, but the energy stored in the inductor and capacitor is dissipated as heat in the resistor. To alleviate energy-inefficiency problems associated with dissipative snubbers, various passive lossless snubbers have been proposed. The concept of such passive lossless snubbers is to reset the energy absorbing circuits by releasing or re-circulating the energy stored to an energy tank, such as an inductor, a capacitor, supply and/or load.
A straightforward approach to resetting the snubber is to use a switching converter, such as a forward or flyback converter, to re-circulate the energy stored in the snubber. The switching action of the main switch is made common to both the main power conversion and snubber energy conversion processes. However, the transformer coupling effect in such converters introduces additional voltage stress across the switch and the leakage inductance of the transformer or coupled inductors also generate undesirable voltage spikes.
Another approach to snubbering is based on using resonant circuits with passive reactive elements and diodes only. The structures of such circuits are simple and can be incorporated readily into existing converters. The absorbed energy is transferred to the source or load or another selected part of the circuit containing the snubber cell through several LC resonant paths created by the main switch and the resonant circuits' diodes.
A typical such snubber cell is described in a publication by M. Ferranti, P. Ferraris, A. Fratta, and F. Profumo et al entitled “Solar energy supply system for induction motors and various loads,” published in Proc. 10th International Telecommunication Energy Conference (INTELEC), vol. 2, pp. 15.7/1-15.7/7, October 1989. The concept of operation illustrated by the snubber cell of this publication is described in connection with a boost converter. The energy stored in the snubber inductor is firstly released to the snubber capacitor after the switch is switched to off. When the switch is switched to on, the stored energy is released to a storage capacitor and then to the load through resonant paths formed by the snubber cell diodes, snubber inductor, snubber capacitor, and the storage capacitor. Some improved circuits with saturable inductors added for reducing reverse recovery current of the main diode have been proposed. However, the voltage generated across the saturable inductor causes extra voltage stress on the switch and thus voltage clamping devices, like lossy zener diodes, have to be added.
An approach to investigating the properties and synthesis of the generalized form of the above category of snubbers has been addressed by a series of publications by K. Smith and K. Smedley. The publications are entitled “Properties and synthesis of passive lossless soft-switching PWM converter,” IEEE Trans. Power Electron., vol. 14, no. 5, pp. 890-899, September 1999, “Lossless passive soft-switching methods for inverters and amplifiers,” IEEE Trans. Power Electron., vol. 1, no. 1, pp. 164-173, January 2000, “Engineering design of lossless passive soft switching methods for PWM converters—Part I. With minimum voltage stress circuit cells,” IEEE Trans. Power Electron., vol. 16, no. 3, pp. 336-344, May 2001, and “Engineering design of lossless passive soft switching methods for PWM converters—Part II. With non-minimum voltage stress circuit cells,” IEEE Trans. Power Electron., vol. 17, no. 6, pp. 864-873, November 2002. In these publications, different sets of minimum voltage stress (MVS) and non-minimum voltage stress (NMVS) snubber circuit cells have been derived. The snubbers with MVS have been found to have narrower soft-switching range than the ones with NMVS.
Although known snubber cells can help reduce switching losses, they typically exhibit at least two of the following limitations:    1) The voltage stress across the switch, particularly in snubbers with NMVS, is high because the variation of the voltage across the snubber capacitor during resonance will generate additional off-state stress on the switch.    2) Soft-switching cannot be ensured at heavy load because the snubber cannot be completely reset before the next energy absorbing process. For example, when the load current is high, the snubber inductor has to take a long time to completely discharge. The storage capacitor in the snubber resetting circuit will be discharging faster than that of the snubber inductor in the resetting process. Then, the switch will not be turned on with ZCS.    3) The current stress on the switch is high because the switch current includes the discharging current of the snubber capacitor and main current flow for energy conversion.